Micro-electromechanical systems (MEMS) devices may include piezoelectric elements formed by thin film processes mechanically coupled to a flexible element responsive to force imparted by a proof mass. In some versions, the piezoelectric elements may have electrodes on top and bottom surfaces and operate in the d31 mode. In other versions, the piezoelectric elements may have at least two electrodes on the top surfaces and operate in the d33 mode. Integrating the piezoelectric elements, the proof mass and the flexible element into the MEMS device may involve a trade-off between fabrication cost of the MEMS device and performance of the piezoelectric element.